Arc between curves on a curved surface

Hello, this is the edge of a curved ramp, I need to meet the start point on top then make the edge continue to the edge below. How do I make an arc between these two interpolated curves?

Right now I’m making a surface as the plane, this is just an extruded curve so that I can have a base plane to draw/project the arc to, but current problem is I can get it to align along the curve automatically (like extractisocurve)

Use Osnap to 1. snap the arc to your start point and 2. any location on the other curve.

Are the two curves planar? Are the end points of the new curve specified or can they move? Does the new curve need to be tangent to the input curves?

Does the new curve need to be a circular arc?

If the new curve does not have to be an arc you could use BlendCrv with

If you have further questions post a .3dm file with your curves.